Brake actuator

ABSTRACT

A brake actuator includes an annular housing ( 10   b ) having first and second opposite radial faces (A and B). Cavities are formed in the housing ( 10   b ), some opening onto face (A) and some onto radial face (B). Two pistons ( 16 ) are disposed in each of the cavities and are movable under the influence of fluid pressure exerted from within the respective cavities to a braking position where the pistons ( 16 ) extend axially away from the cavity. The piston ( 16 ) bear against a backing plate of respective brake pads ( 68 ) forcing the brake pads ( 68 ) against brake discs ( 54  and  56 ). By providing cavities on both faces (A and B) of the housing ( 10   b ), an actuator can simultaneously apply brake pads to two spaced apart rotating discs ( 54  and  56 ). A plurality of actuators can be coupled together to act on a plurality of axially spaced apart brake discs to form a multi-pad multi-brake system.

FIELD OF THE INVENTION

The present invention relates to a brake actuator and, in particular, amulti-piston actuator for use in forcing a plurality of brake padsagainst a rotating body.

BACKGROUND OF THE INVENTION

In order to increase braking power it is known to increase the surfacearea of friction material (eg brake pads or brake shoes) for pressingagainst a rotating body. For example, on heavy vehicles present practiceis to use brake drums with brake shoes. Brake drums are known to havecertain advantages over disc brakes. Major advantage of drum brake overdisc brake is the superior braking force in some situations such as thecontinued application of braking force where temperature variationsoccur while the brake is on. However drum brake assemblies are complexin construction and assembly and generally comprise a large number ofcomponents and parts. As a result of the large number of components thebrake assembly is expensive and time consuming to both assemble and tomount the brake system on the vehicle.

A further problem with drum brakes is that the drum shoe brake liningsoften do not become properly bedded in by wearing against the innersurface of the drum. To achieve good results a near perfect match isrequired between the diameters of the friction lining and the brake drumwhen the brake is applied. If the brake shoe lining is not in a propercontact with the brake drum over its entire surface it will result in aninferior or diminished braking force.

In relation to vehicles with disc brakes in order to improve brakingpower, it is possible to mount for example two sets of callipers about arotating disc. An alternate method is to employ multiple discs whichrotate with but are able to slide axially along a rotating body such asan axle and which are interleaved with fixed discs of friction material.In these multi-disc systems, an actuator such as a piston applies anaxial force on the discs, causing them to press against the frictionmaterial and thus provide a braking effect.

While both the above methods for increasing braking power achieve theirdesired effect, they are not without disadvantage. For example, the areataken up by a brake calliper is relatively large compared to the area ofbrake pad that the calliper can apply to a disc. Therefore there is aninherent inefficiency in terms of the number of callipers that can beapplied about a disc compared to the actual area of brake pad/disccontact. Further, there are inherent space limitations in conventionalvehicles which prohibit the mounting of multiple callipers about a disc.With reference to the abovementioned multi-disc systems, these arerelatively complex systems and also take up a substantial space which isoften not available in production vehicles.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alternate ofbrake actuator which is able to apply friction material at multiplediscreet locations on a rotating body.

According to a first aspect of the present invention there is provided abrake actuator including at least:

an annular housing having first and second opposite radial faces;

a plurality of cavities formed in and about said housing, each of saidcavities opening onto one of said first or second radial faces; and

one or more pistons disposed in each of said cavities, each of saidpistons moveable under influence of fluid pressure within a respectivecavity to a braking position where said piston extends axially away fromsaid respective cavity.

Preferably said housing is provided with a plurality of internalgalleries providing fluid communication between a supply of actuatingfluid and one or more of said cavities.

Preferably said galleries are arranged in one or more sets, each setproviding fluid communication to at least one piston in two or moreseparate cavities.

Preferably said cavities pre evenly spaced about said housing.

Preferably at least one cavity opens on to said first radial face and atleast one further cavity opens onto said second radial face.

Preferably circumferentially adjacent cavities open onto opposite onesof said first and second faces.

In an alternate embodiment, at least two of said cavities are relativelylocated to at least partially overlie each other and open ontorespective opposite ones of said first and second radial faces.

In one embodiment the cavities that open onto said first face areradially offset from the cavities that open onto said second face.

Preferably said brake actuator includes a plurality of piston inserts,each insert demountably retained within a respective cavity, whereineach insert defines at least one cylinder for housing a correspondingpiston.

In one embodiment, when each insert defines more than one cylinder, saidinsert is further provided with a passageway providing fluidcommunication between said cylinders.

However, in an alternate embodiment, when said insert defines more thanone cylinder, said cylinders are fluidly isolated from each other.

Preferably, said housing is further provided with a recess in axialalignment with each of said cavities for receiving a brake pad.

According to a further aspect of the present invention there is provideda multi pad brake system including at least:

a rotor having a first radially extending brake disc, said brake dischaving a first radial braking surface and a second opposite radialbraking surface;

first and second brake actuators, each actuator in accordance with thefirst aspect of the present invention; and,

a plurality of brake pads, said brake pads associated with the pistonsof said first and second brake actuators; said first brake actuatorjuxtaposed relative to said first brake disc so that the brake padsassociated with the pistons of the first brake actuator face said firstbraking surface; said second brake actuator juxtaposed relative to saidfirst brake disc so that the brake pads associated with to the pistonsof the second brake actuator face said second braking surface;

whereby, under influence of fluid pressure in the cavities containingthe pistons with which said brake pads are associated, said brake padsare forced into contact with said first and second braking surfaces.

Preferably said first and second brake actuators are in fluidcommunication with each other.

Preferably said rotor includes a second radially extending brake dischaving respective opposite first and second radial braking surfaces,said first and second brake discs axially spaced apart with said secondbrake actuator disposed therebetween; and wherein said second brakeactuator includes at least one piston on the first radial face of thehousing of the second brake actuator which faces the first brakingsurface of the second brake disc and, at least one piston on the secondradial face of the housing of the second brake actuator which faces thesecond braking surface of the first disc whereby, upon influence offluid pressure within the cavities of said second brake actuator, thebrake pads associated with the pistons of the second brake actuator arepressed into contact with said second braking surface of said first discand said first braking surface of said second disc.

Preferably the pistons on the first radial face of the housing of thesecond brake actuator are radially offset from the pistons on the secondradial face of the housing of the second brake actuator.

Preferably said system includes: a third brake actuator, said thirdbrake actuator in accordance with a first aspect of the presentinvention; and brake pads associated with the pistons of the third brakeactuator; said third brake actuator juxtaposed relative to said secondbrake disc so that the brake pads coupled to the pistons of the thirdbrake actuator face said second braking surface of said second disc.

Preferably said third brake actuator is in fluid communication with saidfirst and second brake actuators.

According to the invention there is also provided a method ofconstructing a multi pad multi disc brake system including the steps of:

-   providing a plurality of brake actuators according to any one of    claims 1 to 12;-   providing a rotor having a plurality of brake discs, mutually    adjacent brake discs being axially aligned with each other and    axially spaced a fixed distance apart, each brake disc having    opposite first and second radial braking surfaces;-   disposing said brake actuators about said rotor so that pistons of    said brake actuators face one of the braking surfaces of each brake    disc;-   associating a brake pad with the pistons in each cavity of said    brake actuators;-   relatively juxtaposing said brake actuators and brake discs so that    under the influence of fluid pressure applied to said cavities said    pistons can force said associated brake pads into contact with said    one of said braking surfaces.

Preferably said method includes providing fluid communication between atleast two of said actuators whereby fluid pressure exerted on actuatingfluid in one of said actuators can be communicated via that one actuatorto a second actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view from the front of a partially assembledbrake actuator according to a first embodiment of the invention;

FIG. 2 is a perspective view from the back of a housing incorporated inthe actuator depicted in FIG. 1;

FIG. 3 is a perspective view of the housing from the front depictinginternal detail;

FIG. 4 is section view of a braking system incorporating a plurality ofactuators;

FIG. 5 is a front perspective view of an insert incorporated in theactuator;

FIG. 6 is a perspective view from the rear of the insert depicted inFIG. 5;

FIG. 7 is a plan view of the insert depicted in FIGS. 5 and 6;

FIG. 8 is a bottom plan view of the insert depicted in FIG. 7; and

FIG. 9 is a view through Section B—B of the insert depicted in FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1–3 of the accompanying drawings, a brake actuator 10in accordance with an embodiment of the present invention includes anannular housing 12 having first and second opposite radial faces A andB.

Three cavities 14A are formed in the housing 12 opening on to face A,and three cavities 14B formed in the housing 12 opening on to face B.The cavities 14A and 14B (hereinafter referred to in general as“cavities 14”) are of the same shape and configuration and are evenlyspaced about respective faces A and B. Further, in this particularembodiment, the cavities 14 are circumferentially staggered about thehousing 10 so that the cavities 14A and 14B do not overlie each other.The cavities 14 are blind cavities in that they do not fully extendthrough the axial thickness of the housing 10.

Two pistons 16 are disposed in each of the cavities 14. (For the sake ofclarity, the pistons 16 are depicted only in the cavity 14A at the “10o'clock” position in FIG. 1.) The pistons 16 are moveable under theinfluence of fluid pressure exerted from within respective cavity 14 toa braking position where the pistons 16 extend axially away from thecavity 14.

With reference to FIG. 4 the pistons 16 bear against a backing plate ofa brake pad 68 for forcing the brake pad against a rotating disc such asbrake discs 54 and 56 of a motor vehicle.

In order to accommodate the brake pads 68 the housing 10 is alsoprovided with a plurality of recesses 18, one of each overlying arespective cavity 14.

By providing cavities 14 on opposite faces A and B of the housing 10, itwill be appreciated that the actuator 10 can operate simultaneously ontwo spaced apart parallel rotating discs.

Referring to FIG. 3, it can be seen that the actuator 10 also includes aplurality of internal galleries 20 providing fluid communication betweena supply of actuating fluid (e.g. brake fluid, not shown) and one ormore of the cavities 14. In this embodiment, a relatively simplearrangement of galleries 20 is provided in which mutuallycircumferentially adjacent ends of adjacent cavities 14 are linked bytwo separate crossed internal galleries 20. The crossing of adjacentgalleries 20 can occur by the galleries 20 extending diagonally relativeto each other but in respective parallel planes. It is however possibleto form the galleries 20 so as to also cross over in respective inclinedplanes.

A fluid inlet port 22 and fluid outlet port 24 are formed axially in thehousing 10 adjacent to each other. Internal gallery 26 provides fluidcommunication between the inlet port 22 and an adjacent cavity 14A whileinternal gallery 28 provides fluid communication between the outlet port24 and the adjacent cavity 14B.

FIGS. 5–9 depict a piston insert 30 for insertion into each of thecavities 14 to define respective cylinders 32 for housing correspondingpistons 16. The insert 30 is provided with a rear face 34 in which isformed a plurality of channels 36 extending between the adjacentcylinders 32. The channels 36 provide fluid communication between theadjacent cylinders 32. Accordingly, by this arrangement, the pistons 16within a particular cavity 14 are subjected to substantially the samefluid pressure.

The thickness of the insert 30 is arranged to be substantially the sameas the depth of the cavities 14 so that when the insert 30 is placedwithin the cavity 14, front face 38 of the insert 30 lies substantiallyflush with the opening of the cavity 14. A pair of holes 40 is formed ona peripheral wall 42 of the insert 30 which is located on a radiallyouter side of the insert 30 when disposed within the cavity 14. Theholes 40 register with corresponding holes 44 formed radially in thehousing 10. Fasteners such as grub screws (not shown) threadingly engagethe holes 40 and 44 to secure the inserts 30 within their respectivecavities 14.

A circumferential groove 46 is formed about an inner circumferentialsurface of each cylinder 32 near the front face 38 for seating a seal(not shown).

FIG. 4 illustrates a multi pad brake system 50 which includes a rotor 52and three brake actuators 10 a, 10 b and 10 c. The actuators 10 a, 10 band 10 c are in a general form similar to the actuator 10 depicted inFIGS. 1–3. The rotor 52 includes first and second radially extendingbrake discs 54 and 56. The disc 54 has first and second opposite radialbraking surfaces 58 and 60 respectively. Likewise, the brake disc 56 hasopposite first and second radial braking surfaces 62 and 64respectively. The brake disc 54 is formed integrally with the rotor 52while the disc 54 is in the form of a separate annular disc which isattached by way of bolts (not shown) to the rotor 52.

Each of the actuators 10 a, 10 b and 10 c is provided with a pluralityof cavities 14 which house pistons 16 in a similar manner as describedin relation to the actuator 10 illustrated in FIGS. 1–3. However, in thesystem 50 depicted in FIG. 4, the actuators 10 a and 10 c are providedwith cavities 14 which open on to respective faces A only of thehousings 12 a and 12 c of the corresponding actuators 10 a and 10 c. Onthe other hand, the actuator 10 b is provided with cavities 14 that openon to both opposite faces A and B of the corresponding housing 12 b.Further the cavities 14 and thus the pistons 16 on face A of actuator 10b are radially offset relative to those on face B. Associated with, orotherwise coupled to, the pistons 16 are brake pads 68.

The first actuator 10 a is juxtaposed relative to the first brake disc54 so that the brake pads 68 coupled to the pistons 16 of the actuator10 a face the first braking surface 58.

The second brake actuator 10 b is disposed between the brake discs 54and 56 and is juxtaposed so that the brake pads 68 coupled to thepistons 16 on face B of the actuator 10 b face the braking surface 60 ofthe disc 54 while the brake pads 68 coupled to the pistons 16 on face Aof the actuator 10 b face the braking surface 62 of the second brakedisc 56.

The brake pads 68 coupled to the pistons 16 of the actuator 10 c facethe second braking surface 64 of the second brake disc 56. When brakefluid (not shown) within the cavities 14 which house the pistons 16 ispressurized, the pads 68 are pressed into contact with the brakingsurface 58 and 60 of brake disc 54 and surfaces 62 and 64 of brake disc56.

The actuators 10 a, 10 b and 10 c are mechanically coupled together forexample by way of bolts (not shown). More significantly however theactuators 10 a, 10 b and 10 c are in fluid communication with eachother. This is achieved by placing the internal galleries 20 a ofactuator 10 a in fluid communication with the galleries 20 b ofactuators 10 b and in turn, the galleries 20 b in fluid communicationwith the galleries 20 c of actuator 10 c. This can be achieved bycoupling the outlet port 24 of actuator 10 a with the inlet port 22 ofactuator 10 b and the outlet port 24 of actuator 10 b with the inletport 22 of actuator 10 c. Hydraulic fluid is provided to the pistons 16of actuators 10 a, 10 b and 10 c through the inlet port 22 of actuator10 a. FIG. 4 further depicts brake fluid bleeding ports 25 and 27.

For simplicity, the inserts 30 are not depicted in FIG. 4. However, FIG.4 further depicts the inclusion of heat shielding seals 70 which havebeen snap fitted to the ends of pistons 16 adjacent the correspondingpads 68. The seals 70 are preferably of the form described inApplicant's co-pending Patent Application No. PR 7395, the contents ofwhich are incorporated herein by way of reference. Second seals in theform of O-rings 72 are further depicted which are seated within theactuators 10 (or more specifically within the inserts 30 not shown).

The system 50 depicted in FIG. 4 can be considered to be a modular multipiston multi disc brake system. The modularity being manifested by theability to axially stack both actuators 10 and brake discs to the rotor52. When multiple brake discs are provided a single brake actuator 10 bhaving pistons on both faces A and B can be located between adjacentbrake discs as exemplified by actuator 10 b in FIG. 4. Alternatively, ifdesired, “single sided” actuators having pistons on one face only can beprovided in a back to back configuration between adjacent discs. Fluidcoupling between adjacent actuators 10 can be achieved by way ofseparate tubular conduits which connect the actuators 10 together oralternately by forming the actuators 10 with integral coupling conduitswhich extend laterally from the ports 22 and 24 for registration withcorresponding ports 22 and 24 of adjacent actuators 10.

From the above description, it will be appreciated that the actuator 10facilitates the application of multiple brake pads at differentlocations on to the same surface of a single disc and moreover that asingle actuator 10 (e.g. actuator 10 b) can apply brake pads to twoaxially spaced adjacent discs. The actuator 10 further facilitates theconstruction of a braking system provided with a plurality of spacedapart rotating discs fixed on an axle with respective actuators 10disposed therebetween. Such a system differs from that described in theBackground of the Invention in that the discs are not required toaxially slide along the axle in order to effect a braking action.

Now that an embodiment of the present invention has been described indetail it will be apparent to those skilled in the relevant arts thatnumerous modifications and variations may be made without departing fromthe basic inventive concepts. For example, in FIGS. 1–3, the housing 12is depicted as being provided with six cavities 14, with three on eachof faces A and B, and with circumferentially adjacent cavities 14 beingstaggered. However, the actuator 10 can be made with all of the cavitiesopening on to one of faces A or B. Alternately, the actuator 10 can beprovided with pairs of cavities partially or fully overlapping axiallyon opposite faces A and B. Further, the actuator 10 can be formed of anynumber of cavities on either face A or B.

In a further modification, the galleries 20 can be arranged in sets toprovide fluid communication to selected pistons 16. Thus, for example,the galleries 20 may be arranged so as to communicate fluid pressurefrom a common master cylinder (not shown) to both pistons 16 in thecavities 14A in the 10 and 2 o'clock positions with reference to FIG. 1so as to act as service brakes, with another set of galleries providingseparate fluid communication between another master cylinder (or othertype of fluid pressure source) and the pistons in the cavity 14 at the 6o'clock position which may then be used as a park or emergency brake.Further, it is possible, if desired, to arrange the galleries 20 and theinserts 30 so that the individual pistons 16 within a particular cavity14 are separately subjected to fluid pressure.

All such modifications and variations together with others that would beobvious to a person of ordinary skill in the relevant arts are deemed tobe within the scope of the present invention the nature of which is tobe determined from the above description, and the appended claims.

1. A brake actuator comprising: an annular housing having first andsecond opposite radial faces; a plurality of cavities formed in andabout said housing, at least one of said cavities opening onto saidfirst radial face, and at least one of said cavities opening onto saidsecond radial face, said annular housing being in the form of anannulus; wherein circumferentially adjacent cavities open onto oppositeones of said first and second faces; and, one or more pistons disposedin each of said cavities, each of said pistons moveable under influenceof fluid pressure within a respective cavity to a braking position wheresaid piston extends axially away from said respective cavity.
 2. Thebrake actuator according to claim 1 wherein said housing is providedwith a plurality of internal galleries providing fluid communicationbetween a supply of actuating fluid and one or more of said cavities. 3.The brake actuator according to claim 2 wherein said galleries arearranged in one or more sets, each set providing fluid communication toat least one piston in two or more separate cavities.
 4. The brakeactuator according to claim 1 wherein said cavities are evenly spacedabout said housing.
 5. The brake actuator according to claim 1 whereinsaid brake actuator includes a plurality of piston inserts, each insertdemountably retained within a respective cavity, wherein each insertdefines at least one cylinder for housing a corresponding piston.
 6. Thebrake actuator according to claim 5 wherein when each insert definesmore than one cylinder, said insert is further provided with apassageway providing fluid communication between said cylinders.
 7. Thebrake actuator according to claim 5 wherein when said insert definesmore than one cylinder, said cylinders are fluidly isolated from eachother.
 8. The brake actuator according to claim 1 wherein said housingis further provided with a recess in axial alignment with each of saidcavities for receiving a brake pad.
 9. A brake actuator comprising: anannular housing having first and second opposite radial faces; aplurality of cavities formed in and about said housing, at least one ofthe cavities opening onto the first radial face and at least one of thecavities opening onto the second radial face wherein at least two ofsaid cavities are relatively located to at least partially overlie eachother and open onto respective opposite ones of said first and secondradial faces; and, one or more pistons disposed in each of saidcavities, each of said pistons moveable under influence of fluidpressure within a respective cavity to a braking position where saidpiston extends axially away from said respective cavity.
 10. A brakeactuator comprising: an annular housing having first and second oppositeradial faces; a plurality of cavities formed in and about said housing,at least one of the cavities opening onto the first radial face and atleast one of the cavities opening onto the second radial face; whereinsaid at least one cavity that opens onto said first radial face isdisposed radially closer to an internal radius of said annular housingthan said at least one cavity that opens onto said second radial face;and, one or more pistons disposed in each of said cavities, each of saidpistons moveable under influence of fluid pressure within a respectivecavity to a braking position where said piston extends axially away fromsaid respective cavity.
 11. A multi pad brake system including at least:a rotor having a first radially extending brake disc, said brake dischaving a first radial braking surface and a second opposite radialbraking surface; first and second brake actuators, each actuatorcomprising: an annular housing having first and second opposite radialfaces; a plurality of cavities formed in and about said housing, each ofsaid cavities opening onto one of said first or second radial faces; oneor more pistons disposed in each of said cavities, each of said pistonsmoveable under influence of fluid pressure within a respective cavity toa braking position where said piston extends axially away from saidrespective cavity; and, a plurality of brake pads, said brake padsassociated with the pistons of said first and second brake actuators;said first brake actuator juxtaposed relative to said first brake discso that the brake pads associated with the pistons of the first brakeactuator face said first braking surface; said second brake actuatorjuxtaposed relative to said first brake disc so that the brake padsassociated with the pistons of the second brake actuator face saidsecond braking surface; whereby, under influence of fluid pressure inthe cavities containing the pistons with which said brake pads areassociated, said brake pads are forced into contact with said first andsecond braking surfaces.
 12. The multi-pad brake system according toclaim 11 wherein said first and second brake actuators are in fluidcommunication with each other.
 13. The multi-pad brake system accordingto claim 11 wherein said rotor includes a second radially extendingbrake disc having respective opposite first and second radial brakingsurfaces, said first and second brake discs axially spaced apart withsaid second brake actuator disposed therebetween; and wherein saidsecond brake actuator includes at least one piston on the first radialface of the housing of the second brake actuator which faces the firstbraking surface of the second brake disc and, at least one piston on thesecond radial face of the housing of the second brake actuator whichfaces the second braking surface of the first disc whereby, uponinfluence of fluid pressure within the cavities of said second brakeactuator, the brake pads associated with the pistons of the second brakeactuator are pressed into contact with said second braking surface ofsaid first disc and said first braking surface of said second disc. 14.The multi-pad brake system according to claim 13 wherein the pistons onthe first radial face of the housing of the second brake actuator areradially offset from the pistons on the second radial face of thehousing of the second brake actuator.
 15. The multi-pad brake systemaccording to claim 13 including a third brake actuator, and brake padsassociated with the pistons of the third brake actuator; said thirdbrake actuator juxtaposed relative to said second brake disc so that thebrake pads coupled to the pistons of the third brake actuator face saidsecond braking surface of said second disc.
 16. The multi-pad brakesystem according to claim 15 wherein said third brake actuator is influid communication with said first and second brake actuators.
 17. Amethod of constructing a multi-pad multi-disc brake system including thesteps of: providing a plurality of brake actuators, each brake actuatorcomprising: an annular housing having first and second opposite radialfaces; a plurality of cavities formed in and about said housing, each ofsaid cavities opening onto one of said first or second radial faces; oneor more pistons disposed in each of said cavities, each of said pistonsmoveable under influence of fluid pressure within a respective cavity toa braking position where said piston extends axially away from saidrespective cavity; providing a rotor having a plurality of brake discs,mutually adjacent brake discs being axially aligned with each other andaxially spaced a fixed distance apart, each brake disc having oppositefirst and second radial braking surfaces; disposing said brake actuatorsabout said rotor so that pistons of said brake actuators face one of thebraking surfaces of each brake disc; associating a brake pad with thepistons in each cavity of said brake actuators; relatively juxtaposingsaid brake actuators and brake discs so that under the influence offluid pressure applied to said cavities said pistons can force saidassociated brake pads into contact with said one of said brakingsurfaces.
 18. The method of claim 17 wherein said method includesproviding fluid communication between at least two of said actuatorswhereby fluid pressure exerted on actuating fluid in one of saidactuators can be communicated via that one actuator to a secondactuator.
 19. A brake actuator comprising: an annular housing havingfirst and second opposite radial faces; a plurality of blind cavitiesformed in and about said housing, at least one of the cavities openingonto the first radial face and at least one of the cavities opening ontothe second radial face; and one or more pistons disposed in each of saidcavities, each of said pistons moveable under influence of fluidpressure within a respective cavity to a braking position where saidpiston extends axially away from said respective cavity.
 20. The brakeactuator according to claim 19 wherein said housing is provided with aplurality of internal galleries providing fluid communication between asupply of actuating fluid and one or more of said cavities.
 21. Thebrake actuator according to claim 20 wherein said galleries are arrangedin one or more sets, each set providing fluid communication to at leastone piston in two or more separate cavities.
 22. The brake actuatoraccording to claim 19 wherein circumferentially adjacent cavities openonto opposite ones of said first and second radial faces.
 23. The brakeactuator according to claim 19 wherein at least two of said cavities arerelatively located to at least partially overlie each other and openonto respective opposite ones of said first and second radial faces. 24.The brake actuator according to claim 19 wherein the at least one cavitythat opens onto one of the first or second radial faces are disposedradially closer to the internal radius of the annual housing than the atleast one cavity that opens onto the other of the second or first radialfaces.
 25. A brake actuator comprising: a single piece annular housinghaving first and second opposite radial faces, a continuous innercircumferential surface between the first and second radial faces and acontinuous outer circumferential surface between the first and secondradial faces, the inner circumferential surface defining a holeextending through a center of the housing and opening onto both thefirst and second radial faces; a plurality of cavities formed in andabout said housing, at least one of the cavities opening onto the firstradial face and at least one of the cavities opening onto the secondradial face; and, one or more pistons disposed in each of said cavities,each of said pistons moveable under influence of fluid pressure within arespective cavity to a braking position where said piston extendsaxially away from said respective cavity.
 26. The brake actuatoraccording to claim 25 wherein said housing is provided with a pluralityof internal galleries providing fluid communication between a supply ofactuating fluid and one or more of said cavities.
 27. The brake actuatoraccording to claim 26 wherein said galleries are arranged in one or moresets, each set providing fluid communication to at least one piston intwo or more separate cavities.
 28. The brake actuator according to claim25 wherein circumferentially adjacent cavities open onto opposite onesof said first and second faces.
 29. The brake actuator according toclaim 25 wherein at least two of said cavities are relatively located toat least partially overlie each other and open onto respective oppositeones of said first and second radial faces.
 30. The brake actuatoraccording to claim 25 wherein the at least one cavity that opens ontoone of the first or second radial faces are disposed radially closer tothe internal radius of the annular housing than the at least one cavitythat opens onto the other of the second or first radial faces.
 31. Abrake actuator comprising: a stator, the stator being formed as anannular housing having first and second opposite radial faces; aplurality of cavities formed in and about said housing, at least one ofthe cavities opening onto the first radial face and at least one of thecavities opening onto the second radial face; and one or more pistonsdisposed in each of said cavities, each of said pistons moveable underinfluence of fluid pressure within a respective cavity to a brakingposition where said piston extends axially away from said respectivecavity.
 32. The brake actuator according to claim 31 wherein saidhousing is provided with a plurality of internal galleries providingfluid communication between a supply of actuating fluid and one or moreof said cavities.
 33. The brake actuator according to claim 32 whereinsaid galleries are arranged in one or more sets, each set providingfluid communication to at least one piston in two or more separatecavities.
 34. The brake actuator according to claim 33 whereincircumferentially adjacent cavities open onto opposite ones of saidfirst and second faces.
 35. The brake actuator according to claim 34wherein at least two of said cavities are relatively located to at leastpartially overlie each other and open onto respective opposite ones ofsaid first and second radial faces.
 36. The brake actuator according toclaim 31 wherein the at least one cavity that opens onto one of thefirst or second radial faces are disposed radially closer to theinternal radius of the annular housing than the at least one cavity thatopens onto the other of the second or first radial faces.